The use of a separator between an anode and cathode in batteries, fuel cells, and electrochemical cells is known. In the past, these separators have been generally porous separators, such as asbestos diaphragms, used to separate reacting chemistry within the cell. Particularly, for example, in diaphragm chlorine generating cells, such a separator functions to restrain back migration of OH.sup.- radicals from a cell compartment containing the cathode to a cell compartment containing the anode. A restriction upon OH.sup.- back migration has been found to significantly decrease overall electric current utilization inefficiencies in operation of the cells associated with a reaction of the OH.sup.- radical at the anode releasing oxygen.
More recently separators based upon an ion exchange copolymer have found increasing application in batteries, fuel cells, and electrochemical cells. One copolymeric ion exchange material finding particular acceptance in electrochemical cells such as chlorine generation cells has been fluorocarbon vinyl ether copolymers known generally as perfluorocarbons and marketed by E. I. duPont under the name Nafion.RTM..
These so-called perfluorocarbons are generally copolymers of two monomers with one monomer being selected from a group including vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkylvinyl ether), tetrafluoroethylene and mixtures thereof.
The second monomer is selected from a group of monomers usually containing an SO.sub.2 F or sulfonyl fluoride group or a COF or carbonyl fluoride group. Examples of such second monomers can be generically represented by the formula CF.sub.2 .dbd.CFR.sub.1 SO.sub.2 F and CF.sub.2 .dbd.CFR.sub.1 COF. R.sub.1 in the generic formula is a bifunctional perfluorinated radical comprising 1 to 8 carbon atoms but occasionally as many as 25 carbon atoms. One restraint upon the generic formula is a general requirement for the presence of at least one fluorine atom on the carbon atom adjacent the --SO.sub.2 F or COF, particularly where the functional group exists as the --(SO.sub.2 NH).sub.m Q form. In this form, Q can be hydrogen or an alkali or alkaline earth metal cation and m is the valence of Q. The R.sub.1 generic formula portion can be of any suitable or conventional configuration, but it has been found preferably that the vinyl radical comonomer join the R.sub.1 group through an ether linkage.
Typical sulfonyl fluoride containing monomers are set forth in U.S. Pat. Nos. 3,282,875; 3,041,317; 3,560,568; 3,718,627 and methods of preparation of intermediate perfluorocarbon copolymers are set forth in U.S. Pat. Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583. These perfluorocarbons generally have pendant SO.sub.2 F based functional groups.
Chlorine cells equipped with separators fabricated from perfluorocarbon copolymers have been utilized to produce a somewhat concentrated caustic product containing quite low residual salt levels. Perfluorocarbon copolymers containing perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comonomer have found particular acceptance in Cl.sub.2 cells.
In chlorine cells using a sodium chloride brine feedstock, one drawback to the use of perfluorocarbon separators having pendant sulfonyl fluoride based functional groups has been a relatively low resistance in desirably thin separators to back migration of caustic including OH.sup.- radicals from the cathode to the anode compartment. This back migration contributes to a lower current utilization efficiency in operating the cell since the OH.sup.- radicals react at the anode to produce by-products. Recently, it has been found that if pendant sulfonyl fluoride based cationic exchange groups adjacent one separator surface were converted to pendant carboxylate groups, the back migration of OH.sup.- radicals in such Cl.sub.2 cells would be significantly reduced. Conversion of sulfonyl fluoride groups to carboxylate groups is discussed in U.S. Pat. No. 4,151,053.
Presently, perfluorocarbon separators are generally fabricated by forming a thin membrane-like sheet under heat and pressure from one of the intermediate copolymers previously described. The ionic exchange capability of the copolymeric membrane is then activated by saponification with a suitable or conventional compound such as a strong caustic.
Generally, such membranes are between 0.5 mil and 150 mil in thickness. Reinforced perfluorocarbon membranes have been fabricated, for example, as shown in U.S. Pat. No. 3,925,135.
Being thin, these membranes, while strongly resistant to the chemical environment within the electrolytic cell, are often subject to physical damage: tears, punctures and flex fatigue cracking. One past proposal has been to repair this physical damage using low equivalent weight copolymer solvated with an alcohol. Such repaired areas have not offered desirable membrane performance characteristics normally associated with higher density copolymeric material, and overall performance of the membrane has declined. These repairs often have achieved less than desirable adhesion to the membrane since mechanical bonding not solvent molding is a significant factor in adhesion.
The use of alcohols to solvate particularly low equivalent weight perfluorocarbon copolymers is known. However, as yet, proposals for formation of perfluorocarbon composite electrodes and for solvent welding the composites to perfluorocarbon membranes where the perfluorocarbons are of relatively elevated equivalent weights desirable in, for example, chlorine cells, have not proven satisfactory. Dissatisfaction has been at least partly due to a lack of suitable techniques for dispersing or solvating in part these higher equivalent weight perfluorocarbons.
In another proposal, heat and pressure have been utilized to adhere a patch of the higher density copolymeric material to the damaged membrane portion. These repairs have met with limited success since heat necessary to fuse reliably the patch to the damaged area can impair cation exchange functionality of the copolymeric material.